Since the experimental version of UE5 I have had problems regarding scalability and performance, but it seemed like the main culprit was r.ScreenPercentage not working, once in a while I would return to the engine and check if this had been solved or if a page of the official documentation was created explaining the reason why this was happening.
Here there go a couple of ways to fix this behaviour for several cases, depending on what antialiasing(AA)/upscaling solution you are using.
By default when you create a new project the default AA solution will be Temporal Super Resolution(TSR), when you use the resolution slider or use r.ScreenPercentage you may not see any change in resolution, but it seems that by using r.TSR.History.ScreenPercentage
( example: r.TSR.History.ScreenPercentage 50) you can control this value an solve this issue. In this case the problem seems to be that TSR is not linked to r.ScreenPercentage, but rather to r.TSR.History.ScreenPercentage.
If you changed your AA solution to Temporal Antialiasing(TAA), by default TAA Upsampling will get turned on, and you will have to run r.TemporalAA.Upsampling 0 to see ScreenPercentage working. In this case the problem seems to be that TAA Upsampling is upscaling the viewport to the full screen resolution, so no perceivable difference can be noticed between certain percentage and 100%, but depending on your hardware this may be too resource intensive, for that reason disabling TAA Upsampling will get you more fps.
Personally I used r.ScreenPercentage to be able to work with the most fps I can get and plugins like DLSS, FSR and XESS can help to get a higher visual fidelity while getting high fps.
For UE5 an AMD FSR plugin compiled by user lucoiso can be found here GitHub - lucoiso/fsr-amd-ue: Adapted version of the FidelityFX Super Resolution for Unreal Engine 5..
In the Unreal Marketplace a DLSS plugin for UE4/UE5 can be found NVIDIA DLSS in Code Plugins - UE Marketplace
And for XESS you can check this link Intel DevMesh where in case you fulfill the requirements you could be considered for an early XESS sample, or you could wait in case a public release is made available.